Engineering skyrmions and emergent monopoles in topological spin crystals

Abstract

Spin structures with a non-trivial topology can emerge through the complex interplay of underlying magnetic interactions. Representative examples are magnetic skyrmions and hedgehogs observed in various materials. Although the most typical size of a skyrmion is 10–100 nm, there has been remarkable progress in the discovery of ultra-small (<3 nm) skyrmions and hedgehogs in the last few years. The dense topological spin crystals not only hold promise for technological applications but also provide a good arena to explore gigantic responses from emergent electromagnetic fields or Berry curvature. Here, we review design principles as well as electronic functions of versatile topological spin crystals, highlighting the distinct properties between skyrmion- and hedgehog-lattice states. Among them, unconventional outcomes from hedgehog-lattice states, such as their formation mechanisms and transport properties induced by the emergent magnetic monopoles, are discussed. The manipulation of such topological spin crystals, based on the strong couplings between topology and spin-charge-lattice degrees of freedom, may pave the way for electronics emerging in the near future.

Spin Hamiltonians in Magnets: Theories and Computations

“…Recently, more and more novel magnetic states such as spin glasses [14,15], spin ice [16,17], spin liquid [18][19][20][21][22], and skyrmions [23][24][25][26][27][28] were found, revealing both theoretical and practical significance. For example, hedgehogs and anti-hedgehogs can be seen as the sources (monopoles) and the sinks (antimonopoles) of the emergent magnetic fields of topological spin textures [29], while magnetic skyrmions have shown promise as ultradense information carriers and logic devices [24].…”

Giant anomalous Hall effect from spin-chirality scattering in a chiral magnet

“…Here we report the observation of a giant AHE in a chiral magnet MnGe thin film, where the giant Hall angle ( %) leads to a Hall conductivity reaching Ω −1 cm −1 , being two orders of magnitude larger than the intrinsic AHE, even with a moderate longitudinal conductivity ( Ω −1 cm −1 ). The target material is the epitaxially grown thin films of MnGe 22 , a member of the B20-type chiral magnets hosting topological spin textures 23 , 24 . The crystal structure belongs to the non-centrosymmetric space group P 2 1 3, where the lattice chirality is characterized by the stacking direction of the atoms as viewed from [111] axis (only one enantiomeric form is shown in Fig.…”

“…1a ). Since the lack of inversion symmetry allows Dzyaloshinskii–Moriya interaction (DMI), twisted spin structures such as helical or non-coplanar structures are often observed in B20-type magnets 23 , 24 . Among them, MnGe hosts unique magnetic textures composed of spin hedgehogs and anti-hedgehogs.…”

“…Among them, MnGe hosts unique magnetic textures composed of spin hedgehogs and anti-hedgehogs. In a bulk sample, those magnetic defects form a short-period lattice (~2.8 nm) with large magnetic fluctuations even persisting in the nominally ferromagnetic (FM) region 24 . The hedgehog lattice is deformed to a rhombohedral form (Supplementary Fig.…”

“…We assume that the difference in the Hall angle between MnGe and KV 3 Sb 5 may be related to the resonance condition for the spin-chirality skew-scattering. For MnGe, the unusually short magnetic period ( nm) has been discussed in terms of the conduction-electron mediated mechanisms 24 , 28 , 29 such as the Rudermann–Kittel–Kasuya–Yosida (RKKY) interaction; this can automatically tune the size of the spin cluster to the typical size of , satisfying the resonance condition. Hence the RKKY magnets or the metallic spin-glass systems would be the promising candidates for achieving a large Hall angle.…”